In a typical thermal printer, a web-type carrier containing a repeating series of spaced frames of different colored heat transferable dyes is spooled on a supply spool. The carrier is paid out from the supply spool and rewound on a take-up spool. The carrier moves through a nip formed between a thermal print head and a dye-absorbing receiver. The receiver in turn is supported by a platen in the form of a drum. The print head engages the dye-carrier and presses it against the receiver. The receiver may, for example, be coated paper and the print head is formed of, for example, a plurality of heating elements. When a particular heating element is energized, it is heated. In the presence of heat and pressure, dye from the carrier is caused to transfer to the dye-receiver. The density or darkness of the printed color dye is a function of the energy delivered from the heating element to the carrier. These types of thermal printers offer the advantage of "true continuous tone" dye density transfer. This result is obtained by varying the energy applied to each heating element, yielding a variable dye density image pixel on the receiver.
The web-type carrier often includes a repeating series of spaced yellow, magenta and cyan dyes frames. The carrier is typically formed of a very thin, flexible dye carrying member having a thickness that can be on the order of 1/4 mil. At the beginning of the print cycle, the head must be locked off the drum to allow a cut-sheet dye-receiver to be wrapped onto the drum and advanced under the head. This pre-printing process requires that the drum turn without the head or dye-carrier in contact with drum. To begin printing, the first dye patch, typically yellow, is advanced to a position under the print head. The print head is lowered to apply pressure on the carrier-donor sandwich as the platen turns. The media sandwich slides under the print head and the heating elements are selectively energized to form a row of yellow image pixels under the print head. The drum turns to generate successive rows of the yellow portion of the final image. When the yellow portion of the image has been deposited, the head is lifted to reposition the dye-receiver for the next color frame. The dye-carrier is controlled during the repositioning so that the next dry frame, for example magenta, is positioned under the print head. When the printer is ready for the second dye frame, the head is lowered to reestablish contact with the media, and the next color plane is deposited on the receiver. The process of head lift, receiver repositioning and preparation for the next dye patch is repeated for the final dye-frame, in this case the cyan frame. The three dyes are blended during the deposition process to generate a full-color image. After the three color portions of the image have been deposited, the printing process is completed. The head must be lifted again to allow the platen to turn and eject the completed image. The head must continue to be held up to reload the platen for the next cut sheet dye-receiver.
The process of applying the head to the drum must be done in a manner that allows the head to be positioned accuratley, repeatedly, and with uniform pressure across the platen to provide a high-quality print. The thermal head's linear array of heating elements should be positioned tangent to the drum and centered radially over the drum surface. In addition, the heating element array should be pressed against the drum surface with uniform force. Because manufactured parts vary from perfect dimension, the mechanism should be assigned to minimize the effect of these dimensional errors on print quality. The accuracy of the head position is ensured by minimizing the number of components between the drum and the head and in the shape and nature of each part's features. If this accuracy cannot be built into the head support mechanism, adjustments must be built in. Such adjustments add to the complexity and expense of the assembly. The repeatability of the mechanism is guaranteed if the head returns to the same position after a lift-and-lower cycle. If the head doesn't return to the same position for each of the dye layers, the resolution of the image will be degraded. It is preferable that the head must not only lift off of the drum during the printing, but that the head should be moved significantly out of the way for service procedures.